The present invention relates generally to microwave ovens including supplementary electrical resistance browning elements and, more particularly, to such an oven which is adapted for operation from an approximately 1500 watt electric power source and which employs an electrical resistance browning element having a relatively high thermal mass.
Ovens employing microwave energy to rapidly cook food have come into widespread use in recent years. While microwave cooking generally has the advantage of being faster than conventional cooking, it has long been recognized that conventional cooking is superior in certain respects. In particular, for some types of food, microwave cooking is considered unsatisfactory by many people for the reason that there is usually only a slight surface browning effect, especially where a relatively short cooking time is employed.
To realize the benefits of both methods, a number of combination microwave and conventional cooking ovens have been proposed and commercially produced. These ovens, as their name implies, combine in a single cavity the capability of microwave cooking and conventional cooking by electrical resistance heating. The microwave cooking capability is provided by a microwave energy generating device such as a magnetron which produces cooking microwaves when energized from a suitable high voltage DC source. For conventional cooking and browning capability, sheathed electrical resistance heating elements, commonly called broil and bake elements, are usually provided at the top and bottom of the cooking cavity respectively.
Several of these combination oven designs have proven to be quite satisfactory in operation and commercially successful. They are typically full-size ovens operated from a 240 volt power source having a current-supplying capability which, for practical purposes, is unlimited. Therefore, simple switching schemes may be employed to selectively energize either the microwave cooking capability, the conventional cooking capability, or both capabilities simultaneously. Many thousands of watts of power are available from the power source, and this is sufficient to heat a domestic sized cooking oven in any manner desired.
More recently, so-called countertop microwave ovens have been introduced. These ovens typically have a somewhat smaller cooking cavity compared to a full-size conventional oven, and are designed for operation from a 115 volt, 15 amp household branch circuit. To meet UL requirements, an appliance designed for operation from such a power source is limited to a maximum steady state requirement of 13.5 amperes. This corresponds to approximately 1550 watts. As explained next, this limited power source capability results in some particular problems.
A typical microwave energy generating system intended for a countertop microwave oven requires a major portion of this available power. Such a typical system comprises a magnetron which produces between 400 and 600 watts of output power at a frequency of 2450 MHz, and a suitable power supply for the magnetron. A typical microwave energy generating system has an energy conversion efficiency in the order of 50%. In addition to the microwave energy generating system, a practical microwave oven includes a number of low power load devices such as lamps, fan motors, and control circuitry. As a typical example, altogether one particular commercially-produced countertop microwave oven model draws approximately 11.2 amps RMS from a 115 volt line for microwave cooking alone. This corresponds to approximately 1300 watts.
For effective and reasonable rapid browning, the watts density over the area of the food covered by a supplementary electrical resistance browning element should be approximately 20 watts per square inch. With 1200 watts of available browning power, approximately 60 square inches could be covered by radiation from such a browning element. Even 60 square inches is a relatively small area, and any decrease in available browner power would reduce the amount of area even further. As a result, substantially all of the limited available power should be supplied to the browning element.
Therefore, for an oven designed for operation from a 115 volt, 15 amp household branch circuit, as a practical matter the limited power available precludes the simultaneous energization of the microwave energy generating system and the supplementary electrical resistance browning units at their respective full rated power levels, which, particularly in the case of the browning element, is required for effective operation.
In answer to this practical limitation on available power, designers of countertop microwave ovens intended for operation from a power source insufficient to supply both the microwave and electrical resistance browning capabilities simultaneously at their respective full rated power levels have resorted to a two-step cooking procedure whereby cooking by microwave energy is accomplished first, with the electrical resistance browning element de-energized. Next the microwave source is de-energized and the electrical resistance browning element is energized for the remainder of the cooking cycle.
As an alternative to a seperate electrically energized heating element for browning, a number of special utensils have been proposed and commercially produced to effect browning when used in a microwave oven. These utensils comprise an element, for example a thin resistive film applied to an undersurface of the utensil, which element has the capability of absorbing some of the microwave energy available in the cooking cavity and converting the same to heat. The utensil itself becomes sufficiently hot for browning or searing. In a similar vein, devices have been proposed which alter the electromagnetic energy field within the cooking cavity so as to produce near field dielectric heating for improved surface browning. It will be appreciated that while such devices are beneficial with certain foods, the microwave energy they absorb is then unavailable for direct heating of the food. Additionally, they are not as efficient as direct electrical resistance heating because the less-than-100% energy conversion efficiency of the microwave energy generating system must be taken into account.
While not directly related to browning, an important feature included in many microwave ovens is a variable microwave power level control. Variable power level control provides flexibility in cooking various types of food, including thawing frozen foods at a reduced power level. One particular power level control scheme which is employed in microwave ovens is duty cycle power level control whereby the microwave energy source is repetitively switched from full OFF to full ON, with the duty cycle under control of the user of the oven. In this way, the time averaged rate of microwave heating can be effectively controlled. The repetition period may vary from in the order of one second for fully electronic duty cycle power level controllers, to in the order of thirty seconds for electromechanical cam operated duty cycle power level controllers.
In accordance with the invention which is the subject matter of the above-mentioned Dills U.S. Pat. No. 4,188,520, effective microwave and electrical resistance heating is accomplished concurrently by a time ratio control system which alternately energizes the microwave energy generating system and the electrical resistance heating system a plurality of times during each cooking operation. This in effect time shares the available power. Actual cooking tests have shown that this gives superior results when compared to the two-step cooking process. One reason for the superior results is that during those periods when the microwave energy source is de-energized and the electrical resistance heating system is energized, the temperature throughout the body of the food being heated is given time to equalize. As is known, many microwave cooking ovens do not have perfectly uniform microwave energy distribution within the cavity, and as a result hot spots and cold spots within the body of food are produced. A common microwave cooking technique is to allow a waiting or "equalization" period during which heat flows from warmer to cooler regions within the body of food. When the time sharing concept is employed, gradual equalization occurs at a number of times during the cooking cycle. Less time is required for final equalization at the end of a cooking operation, and, more importantly, short term temperature differentials are minimized so that the presence of slightly overcooked regions is minimized.
Another benefit of the time sharing approach is partly cosmetic in nature and is apparent when a user observes the food while it is cooking. During conventional cooking, food gradually browns throughout the cooking process. However, with the prior art two-step cooking process employed with microwave oven browner systems, browning does not occur until near the end. With time sharing, browning and cooking progress more nearly resembles that which occurs during conventional cooking, and the result is visually more pleasing than the "two-step" approach. Further, it is believed that the actual cooking results are more similar to that which is achieved in conventional cooking.
The present invention is an improvement of the Dills invention, which improvement relates to the specific case where the electrical resistance heating element is an infrared radiant browning element comprising a sheathed electrical resistance heating unit which inherently has a relatively high thermal mass. Such a heating unit generally comprises a heating element in the form of a spiraled electrical resistance wire encased in an elongated, ceramic-filled metal outer sheath which is electrically conductive and preferably is grounded for safety and to avoid the absorption of microwave energy. The ceramic material transmits heat, but is an electrical insulator. A suitable ceramic material is magnesium oxide. Thus, the outer sheath becomes hot, but normally remains electrically insulated from the heating element. The design of such heaters is highly developed, and at present they are rugged, highly reliable, and, due to the grounded electrically conductive outer sheath, are relatively safe. Thus, in many respects they are an ideal heating element for use in ovens, and in fact are widely so used.
In accordance with the present invention, a specific shortcoming of a sheathed electrical resistance heating unit having a relatively high thermal mass when used as a browning element in a time sharing system such as is disclosed in the Dills U.S. Pat. No. 4,188,520 is recognized and effectively alleviated.